The present invention relates in general to control circuits which require high gain, low saturation voltage, low holding current, high voltage and low leakage in the OFF condition, and fast response time.
The two most commonly used solid-state control devices are the SCR or thyristor, and the power transistor. The thyristor is a high gain, high current and high voltage control device, but it is difficult and slow to turn off. The typical turn-off of a thyristor is to allow the load current to go to zero, or to use a commutating circuit to starve the thyristor anode of current. The power transistor is easy to turn off at some speeds, but requires a large drive current to saturate. The drive current is usually supplied by a Darlington drive transistor which reduces the control current to the base, but raises the voltage across both transistors and slows down the control effected by such transistors. This system can be pulled out of saturation by a surge current of the load which exceeds the drive capability of the transistors. Also, high voltage, high gain transistors are expensive compared to high voltage, low gain transistors and thyristors. The use of the thyristor as a remote base PNP transistor is described in the General Electric SCR manual, Third Edition, 1964, pp. 10-11. Using an SCR in this mode makes it act as a high voltage, low gain PNP transistor. The gate becomes the emitter, the cathode becomes the base, and the anode becomes the collector. This configuration has been known for about 20 years, but has been little used.
The two-transistor analogy of the PNP structure of a thyristor is also described in this Third Edition of the General Electric SCR manual, at page 5. This shows that the PNPN structure may be visualized as consisting of two transistors, a PNP and an NPN, interconnected to form a regenerative feedback pair. That regenerative feedback pair of two opposite types of transistors has been recognized as a latch circuit, e.g., in U.S. Pat. No. 3,902,079, issued Aug. 26, 1975 to Adel A. Ahmed. This PNPN structure is also used in a gate turn-off device. However, in both the thyristor and gate turn-off device, the device will not turn off by merely returning the voltage of the gate to the voltage of the cathode, but the gate must be driven negative relative to the cathode. In the aforementioned patent, the two transistors are disclosed as being of the same size and current handling capability, and are intended to be used in a low power level logic circuit. That patent states that the turn-off may be achieved by removing the operating voltage from the circuit, or by interrupting the base current flow to either transistor.
The problem to be solved, therefore, is how to achieve a simple control circuit which will operate on a wide range of operating voltages, will have high gain, low saturation voltage, low holding current, low initiation current, and fast response time. Also, a circuit is desired which may be operated in the saturated mode and be easily turned on and easily turned off, or which may be operated in the linear control mode in an intermediate range between regenerative conduction and degenerative non-conduction.